By harnessing the monsoon tide that gushed from the Kulen Hills, Angkor and its rulers flourished. From the era of Jayavarman II, who laid the kingdom's foundations in the early 800s, the empire's growth depended on bumper rice harvests. Throughout southern Asia, perhaps only the ancient cities of Anuradhapura and Polonnaruwa in Sri Lanka and their famed reservoirs could compare to Angkor's ability to guarantee a steady water supply.

That reliability required massive feats of engineering, including a reservoir called the West Baray that's five miles long and 1.5 miles wide. To build this third and most sophisticated of Angkor's large reservoirs a thousand years ago, as many as 200,000 Khmer workers may have been needed to pile up nearly 16 million cubic yards of soil in embankments 300 feet wide and three stories tall. To this day the rectangular reservoir, or baray, is fed by water diverted from the Siem Reap River.

The first scholar to appreciate the scale of Angkor's waterworks was Bernard-Philippe Groslier, an archaeologist with the French School of Asian Studies (EFEO). In a landmark 1979 treatise, he envisioned Angkor as a "hydraulic city." The great barays, he argued, served two purposes: to symbolize the primeval sea of Hindu cosmogony and to irrigate rice fields. Unfortunately, Groslier could not pursue this concept further. Cambodia's civil war, the brutal reign of the Khmer Rouge, and the ouster of the regime by Vietnamese forces in 1979 turned Angkor into a no-go zone for two decades. After Vietnamese troops withdrew, looters descended on Angkor, swiping statues and even chiseling off bas-reliefs.

When Christophe Pottier, an architect and archaeologist, reopened EFEO's research station at Angkor in 1992, the first priority was helping Cambodia restore dilapidated and pillaged temples. But Pottier was drawn to the wilderness beyond the temple walls. He spent months crisscrossing the southern half of Greater Angkor on motorbike and foot, mapping once hidden house mounds and shrines near artificial ponds called water tanks. (Lingering lawlessness deterred Pottier from surveying the northern half.) Then, in 2000, Fletcher and his colleague Damian Evans laid hands on NASA radar images of Angkor. They were a revelation: The University of Sydney team, working with EFEO and APSARA, the Cambodian agency that manages Angkor, found vestiges of many more settlements, canals, and water tanks, particularly in Angkor's inaccessible areas. Donald Cooney's ultralight flights have helped Fletcher and Pottier, now a co-director of the Greater Angkor Project, examine these features in finer detail. Crucially, they found inlets and outlets to the barays, ending a debate catalyzed by Groslier's work about whether the colossal reservoirs were used solely for religious rituals or for irrigation. The clear answer is both.